With the advent of digital television, it has become increasingly important that transmission lines carrying broadcast signals to broadcast antennas convey the signals with minimal attenuation and signal distortion. Conventional transmission lines of the broadcast caliber are usually coaxial in nature and very long, being fabricated from joining several smaller coaxial transmission lines together. To maintain the necessary separation between the inner conductor and outer conductor of the coaxial transmission line, series of insulating supports are interspersed within the line at specified locations. The insulating support structures in the coaxial transmission line are subject to several significant loads over their life cycle.
Lines mounted horizontally subject all the supports to a nominal radial load that is proportional to the weight of the inner conductor divided by the number of supporting insulating supports. Lines mounted vertically subject the first support in each section to an axial load at least equal to the weight of the inner conductor plus the forces encountered during the normal differential expansion which occurs between the inner conductor and the outer conductor during power up or power down. Accordingly, for transmission lines that are vertically mounted, the first insulating support or anchor support is subject to a higher load due to axial forces in the transmission line than the load due to radial forces on the individual supports.
Other forms of loads may affect the transmission line such as those associated with the transportation and the installation of the line. For example, it should be understood that radial loads on the individual insulating supports are typically a fraction of the shock load (due to the inertia of the inner conductor) while the axial load on the anchor or end insulator support will equal the entire shock load due to the mast of the inner conductor. Also, there is an axial force called the insertion force associated with the process of forming the connection between successive sections of lines. Again, the axial forces on the anchor or end insulator support is understood to be greatly in excess of the radial loads anticipated on the individual supports.
Also, during the fabrication process, the inner conductor with all its internal support structures must be inserted into the outer conductor and pushed through the entire length of the transmission line section to arrive at its final position. Due to variations or irregularities in the outer conductor diameter or interior finish, for example, the insulating supports are subject to axial forces up to a maximum of the total insertion force (in the case where a single insulating support is the cause of the resistance). These loads are different from the radial loads typically encountered by the inner supports when under normal operating conditions. Therefore, these fabrication loads can be very large and primarily axially directed rather than radially (though there may be an occasional significant radial component).
It should be apparent from the above that the insulating supports' greatest force loading will be due to axial forces rather than conventionally imagined radial forces. In this regard, conventional state-of-the art practices for supporting the inner conductor of rigid coaxial transmission lines have generally been of two types: dielectric pins oriented radially outward from the inner conductor or a disc of dielectric material or puck modified to have mass removed from the interior while retaining sufficient strength to perform the supporting function. These objects are manufactured from an electrically insulating material, generally either of ceramic or plastic composition. The radial forces are carried as compressive loads while axial loads are carried as bending loads, like a beam.
Conventional approaches to addressing the radial and axial loads rising from fabrication and supporting the transmission line have been directed to simply adding to the axial thickness or more material to the supports to accommodate the additional stresses. While the added mass may be effectively compensated for, on the average, by reducing the diameter of the inner section, manufacturing variations become proportionally greater increasing the perturbations of signals in the transmission line.
Therefore, there has been a long standing need in the transmission line community for providing systems and methods for a center conductor support implementation that reduces support mass while providing transmission line integrity.